Current hand tools are arranged with a strain gauge to measure the twisting force applied to the handle tool. In the prior art the sensitivity of the strain gauge is not so sensitive so that derived stresses are not precise and thus users cannot apply proper force according to the values. As a result, it is possible that the handle tool is destroyed or the screw means cannot be well fixed.
To improve above mentioned defect, a round groove is formed on the handle tool and the strain gauge is installed in the groove so that stresses are concentrated upon the groove and the strain gauge can have precise values. However the stresses concentrate upon the wall of the groove, but the groove is round so that the strain gauge cannot fully sense the twisting force. In fact, the strain gauge is installed at the radial direction of the round groove. Thereby it cannot effectively improve the defect in the prior art.
In another improvement, U.S. Pat. No. 5,503,028, the strain gauge is installed at an outer side of the handle tool although a hole is formed. Thus it still cannot get a precise value.